The present invention relates to miniaturized antennas suitable for communication systems including cellular telephones and more particularly to reducing the size of such antennas while still providing an acceptable antenna loading mechanism.
Attempts have been made in the prior art to miniaturize antennas for communications. FIG. 1A for example depicts an end-loaded shortened dipole antenna 10 with a meanderline counterpoise 20. A commercially available antenna 10 such as shown in FIG. 1A suitable for cellular telephony is marketed by Radio Shack Corp. The size of antenna 10 may be compared to the enlarged U.S. quarter, shown in FIG. 1B, the enlargement being the same for FIGS. 1A and 1B. A common resonant frequency for the prior art antenna of FIG. 1A is about 870 MHz.
FIG. 1C depicts antenna 10 used with a cellular telephone 30. While antennas such as antenna 10 do function, they are several cm in length or must be pulled-out to a length of several cm. This length makes the antenna and/or cellular telephone (or other transceiver device) somewhat vulnerable to breakage. Clearly a smaller version of a cellular telephone-type antenna would be bene-As described in the following sections, fractal patterns are preferably used with the present invention. By way of further background, applicant refers to and incorporates herein by reference his PCT patent application PCT/US96/13086, international filing date 8 August 1996, priority date 9 August 1995, entitled xe2x80x9cFractal Antennas and Resonators, and Loading Elementsxe2x80x9d.
The present invention provides an antenna configuration comprising a flexible substrate having spaced-apart first and second surfaces. A conductive pattern is formed on the first surface, the pattern preferably defining complex geometry such as a fractal of first or higher iteration. One portion of the complex pattern defines a feed-point to which RF energy may be coupled or received. (Preferably the other feedpoint will be a groundplane associated with the environment with which the antenna is used, for example the interior shell of a cellular telephone.) The frequency characteristics of the antenna may be tuned by varying the iteration and/or shape of the fractal.
More preferably, tuning is facilitated by disposing a conductive patch spaced-apart by about the substrate thickness from the complex pattern. The patch may be a small square or rectangle or other shape. The patch xe2x80x9cfloatsxe2x80x9d electrically in that it is not directly coupled to any feedline. Instead, the patch acts as a capacitive load that can capacitive couple various locations in the complex pattern. The preferably dielectric substrate couples RF current through the substrate thickness. RF current in the complex pattern on the first surface differs in magnitude from location to location at the through-substrate coupling regions.
On one hand, the complex geometry on the first surface contributes an inductive loading. On the other hand, the patch on the second surface contributes a capacitive loading. In combination, the two loading effects produce a monopole that is dimensionally small physically yet is an efficient radiator of RF energy and exhibits a multiband frequency characteristic. Multiple frequency bands of interest may be produced and tailored by the size, configuration, and/or position of the patch relative to the complex pattern, as well as by the complex pattern itself. If desired, the patch can be formed on a separate layer of substrate that is slid or otherwise moved about relative to the location of the complex pattern, to tune characteristics of the antenna.
The preferably flexible substrate(s) may be partially rolled to form a semi-cylindrical or cylindrical shape. The conformally rolled substrate (with complex pattern and patch on the spaced-apart surfaces) may then be inserted into a cylinder and used to replace the xe2x80x9cduckyxe2x80x9d or xe2x80x9cstubbyxe2x80x9d antenna commonly used in cellular telephone or transceiver applications.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings.